A couple of days ago, I read a neat article about the fingerprint all color laser printers and copiers leave when printing. Invisible to the unaided eye under normal light, these markings uniquely identify the printer that manifested the printout. If one combines that data with customer records from the printer manufacturer, one may trace a printout to a particular person or company. Privacy concerns aside, I think it’s a neat technology.

The secret lies not in special inks or complicated obfuscation; rather, it depends on the human eye’s inability to resolve very small yellow dots on a field of white. These dots are spread across the entire page, from edge to edge. They form a pattern that resembles Braille, albeit significantly smaller. I don’t have instruments that can accurately measure the dots, but I estimate that they are 100 microns in diameter with an average pitch of several millimeters. But wait… how can I see them at all?

To see the dots, all one needs is a blue light source, such as found in any of a multitude of modern LED keychains. Print something from a color laser printer then shine the blue light on the page. Thousands of little dots will spring from the page as if by magic!

Similar techniques protect against counterfeiting. Modern money has patterns embedded so that newer copiers, printers, and photo software will not copy/print/manipulate images of currency. Check out an example of the so-called “Eurion Constellation,” then try to find it on one of the new US 20s.



Very clever!